1. Field of the Invention
The present invention relates to a thin film transistor (TFT) array substrate, a method for manufacturing the same and a liquid crystal display device having the TFT array substrate. More particularly, the present invention relates to a TFT array substrate forming a display and capable of enhancing the display quality, a method for manufacturing the same, and a liquid crystal display device having the TFT array substrate.
2. Description of the Related Art
Generally, a reflective type liquid crystal display device uses an ambient light to display an image. Therefore, in a dark place, the reflective type liquid crystal display device may not display an image clearly.
However, a transmissive type liquid crystal display device uses a light generated from a backlight assembly. Therefore, the transmissive type liquid crystal display device displays an image clearly regardless of ambient brightness. However, the transmissive type liquid crystal display consumes much energy to drive the backlight assembly. Therefore, the transmissive type liquid crystal display is inadequate for a portable display device.
A transmissive and reflective type liquid crystal display has the merits of the reflective type and transmissive type liquid crystal display.
FIG. 1 is a cross-sectional view of a conventional transmissive and reflective type liquid crystal display.
Referring to FIG. 1, a conventional transmissive and reflective type liquid crystal display includes a TFT array substrate 10, a color filter substrate 20, a liquid crystal layer 30, an upper quarter wave plate 40, an upper polarizer 50, a lower quarter wave plate 60 and a lower polarizer 70. The TFT array substrate 10 includes a reflective layer 19 having a reflective region and transmissive window 19a. The color filter substrate 20 faces the TFT array substrate 10. The liquid crystal layer 30 is interposed between the TFT array substrate 10 and the color filter substrate 20. The upper quarter wave plate 40 and the upper polarizer 50 are disposed over the color filter substrate 20 in sequence, and the lower quarter wave plate 60 and the lower polarizer 70 are disposed below the TFT array substrate 10 in sequence.
FIGS. 2A and 2B are schematic views showing an operational principle of the conventional transmissive and reflective type liquid crystal display of FIG. 1. FIG. 2A corresponds to a reflective mode operation, and FIG. 2B corresponds to a transmissive mode operation. Especially, the conventional transmissive and reflective type liquid crystal display corresponds to a normally white mode liquid crystal display that shows a white color, when no electric fields are applied to a liquid crystal layer.
Referring to FIG. 2A, during the reflective mode operation, an external light passes through the upper polarizer 50 to form a linearly polarized light. Then, the linearly polarized light passes through the upper quarter wave plate 40 to form a circularly polarized light. The circularly polarized light may be right-handed or left-handed.
The circularly polarized light passes through the liquid crystal layer 30. When electric field is not applied to the liquid crystal (LC) layer 30, the liquid crystal molecules are twisted. In this case, the phase of the circularly polarized light is changed by λ/4 to form a linearly polarized light after the light passes through the LC layer. The linearly polarized light is reflected on the reflective layer 19 and advances the liquid crystal layer 30 again. Then, the phase of the circularly polarized light is changed by λ/4 to form a circularly polarized light again. The circularly polarized light passes through the upper quarter wave plate 40 again to form a linearly polarized light. Then, the linearly polarized light passes through the upper polarizer 50 to display a white color.
However, when electric field is applied to the liquid crystal layer 30, the circularly polarized light coming from the upper quarter wave plate 40 passes through the liquid crystal layer 30 without phase changes, and the light is reflected on the reflective layer 19. After that, the light advances toward the upper quarter wave plate 40 again to pass through the upper quarter wave plate 40. Then, the phase is changed by λ/4 to form a linearly polarized light of which plane of vibration is perpendicular to the upper polarizer 50. The linearly polarized light does not pass through the upper polarizer 50 to display a black color.
Referring to FIG. 2B, during the transmissive mode operation, a light generated from a backlight assembly passes through the lower polarizer 70 to form a linearly polarized light. Then, the linearly polarized light passes through the lower quarter wave plate 60 to form a circularly polarized light. The circularly polarized light passes through the liquid crystal layer 30. When electric field is not applied to the liquid crystal (LC) layer 30, the liquid crystal molecules are twisted. In this case, the phase of the circularly polarized light is changed by λ/4 to form a linearly polarized light after the light passes through the LC layer. The linearly polarized light passes through the upper quarter wave plate 40 to form a circularly polarized light. Then, the circularly polarized light passes through the upper polarizer 50 to display a white color.
However, when electric fields are applied to the liquid crystal layer 30, the circularly polarized light come from the lower quarter wave plate 60 passes through the liquid crystal layer 30 via the transmissive electrode 18. Then, the circularly polarized light passes through the upper quarter wave plate 40 to form a linearly polarized light of which vibration plane is perpendicular to the upper polarizer 50. The linearly polarized light does not pass through the upper polarizer 50 to display a black color.